On Friday, my local store said the mulligan rule being tested at PTORI (Before starting the game, any player who mulligans to less than 7 gets to scry 1) had been implemented into the game proper. But there's been no announcements on wizards.com to that effect, and no source hosting the comprehensive rules reflects this in 103.4. The TO just said that "mtgsalvation confirmed it", but their wiki doesn't, their front page doesn't, and the last relevant forum post I found was from mid-July, before the pro tour actually happened. I know it's looking likely that it'll stick around, but they were pretty clear that only PTORI would be affected for the time being. Am I missing an announcement somewhere?
The new mulligan rule was officially added starting with Battle for Zendikar prereleases on September 26-27, 2015 (or October 9-12, 2015 for Magic Online).
As of this question being asked, it was definitely not a rule yet. In fact the Latest Developments column on August 7 said:
We will be looking at both how the Pro Tour played out, as well as the reactions from the players regarding the rule. I don't know when we will have a decision based on this, but we hope to have new information fairly soon.
Anything official will surely be on the official site, so I wouldn't bother digging on forums or anything - things like this happen in official ways, not just via rumors and forums.